How can I add an unlisted search engine?

I wish to add an additional search engine ( http://bluebellrailway.easysearch.org.uk/ ) that is not listed in the Add ons Search Tools list, nor does it appear in the Mycroft project (though other easysearch options are).

How can I do so?

I wish to add an additional search engine ( http://bluebellrailway.easysearch.org.uk/ ) that is not listed in the Add ons Search Tools list, nor does it appear in the Mycroft project (though other easysearch options are).
How can I do so?